Positive temperature coefficient (PTC) materials exhibit an electrical resistivity (resistance per unit thickness) which is relatively low at a design operating temperature and increases abruptly as the temperature of the material rises above a critical temperature. This property can be used to create devices which protect electrical circuits against over-current conditions.
A PTC circuit over-current protection device generally comprises a layer of PTC material sandwiched between two plates of electrically conductive metal. Electrical leads are attached to each of the plates and are connected to the electrical circuit. At a given operating temperature, there is a maximum steady level of electrical current which can pass from one plate to the other through the PTC material without causing significant resistance heating of the device. This level of current is known as the "pass" or "hold" current. If the current level rises above the hold current, resistance heating causes the temperature of the PTC element to rise above the critical temperature and the resistance of the device increases sharply so that only a very low level of current can pass through the device, effectively opening the circuit.
PTC devices are typically used in place of conventional fuses and/or circuit breakers in various electrical and electronic devices, and are usually mounted on a printed circuit board or otherwise hard-wired into the circuit to be protected. PTC materials include compositions such as conductive polymers and ceramics.